1. Field of the Invention
The present invention relates to an ink-jet printing apparatus and an ink-jet printing method using the apparatus; more specifically, the invention relates to an ink-jet printing apparatus and an ink-jet printing method for carrying out printing of characters, images, or the like on a printing medium such as printing paper, OHP paper using an ink and a liquid which makes a coloring material in the ink insoluble (hereafter called a treatment liquid).
2. Related Background Art
The ink-jet printing method has various advantages such as low noise, a low running cost, capability of high speed printing, easiness of miniaturization of an apparatus, and easiness of colorization and is widely employed for printers, copying machines, or the like. For the printers, an ink to be employed is selected generally in consideration of the printing properties such as ejecting properties and fixing properties and printing qualities such as the bleeding of printed images, optical reflection density, and coloring ability. It is well known that inks can be divided into two types; a dye-based ink and a pigment-based ink; based on the coloring materials contained in inks. Of these, the pigment-based ink is superior in water resistance and light resistance to the dye-based ink and also has advantageous properties such as capability of clear character quality. On the other hand, as compared with the dye-based ink, the pigment-based ink takes time to be fixed on a printing medium and sometimes the abrasion resistance of obtained images after fixing is not sufficient, and depending on the ink ejected out of a nozzle by one ejecting operation, the size of ink dots formed on a printing medium tends to be small. That is, a pigment contained in a pigment-based ink is generally stably dispersed in the ink by mainly utilizing the electric repulsion force of is a polymer dispersant to cancel the intermolecular force which causes agglomeration of the pigment particles. A polymer dispersant is, therefore, necessary to be added to the ink corresponding to the amount of the pigment. In the case where such a pigment-based ink is employed for printing characters on plain paper by ink-jet recording process, owing to the penetration of the solvent, e.g. water, of the ink into the paper and evaporation of the solvent to ambient air, the pigment particles are agglomerated. At that time, the more the quantity of a polymer dispersant is added, the higher the agglomeration force of the ink on paper is heightened. Thus, the diameter of ink dots formed on a printing medium by a prescribed volume of an ink ejected out of an ink-jet head becomes small and the dot shape Is left approximately in a deformed shape formed at the time of impacting on the paper. Therefore, in order to obtain sufficient recording density to form images and ink dots with a dot diameter sufficient for recording without white stripes or the like, the ejection volume of an ink from an ink jet head is necessarily adjusted to be high. Even if such adjustment is carried out, jointly with the deterioration of the penetration property in paper attributed to the strong agglomeration force of pigment particles on which a polymer dispersant is adsorbed, fixation of an ink on a printing medium is delayed or the abrasion resistance of the recorded images is sometimes decreased.
In order to enlarge the dot diameter and improve the fixing property, it is considered to add a penetrant to an ink for the purpose of improvement of penetration properties of the ink In a printing medium. However, that is sometimes accompanied with undesirable phenomena such as deterioration of the dot shape (deterioration of the peripheral shapes of dots, so called feathering), penetration of the ink to the back side of paper (so called back-through) for the purpose of high quality recorded images. Further, since a coloring material penetrates into the inside of a printing medium, the optical density (OD) of ink dots is not so much increased even though the dot diameter becomes relatively large.
Furthermore, an ink containing a self-dispersing pigment has been proposed, and presumably because of weak agglomeration force of the pigment on paper as compared with that of the above described pigment dispersed in an ink by a dispersant, the ink can provide dots with an enlarged dot diameter but it is not yet sufficient.
As mentioned above, the studies are still on the way to develop a printing method and apparatus capable of providing at a satisfactorily high level various properties, e.g. fixing property of an ink, enlargement of the ink dot diameter, evenness of the density in ink dots, high optical density of ink dots themselves, or the like, which influence the quality of recorded images.
Meanwhile, with the purpose of further improvement of printing quality and image quality in the ink-jet printing technique (for example, improvement of water resistance and optical density (OD) of images on a printing medium), methods for providing an ink and a treatment liquid capable of reacting with the ink to a printing medium so as to cause a reaction of the ink and the treatment liquid with each other in the printing medium have been proposed and practically used.
Facts and Findings
The inventors of the present invention have studied the ink-jet printing technique in which a pigment-based ink and a treatment liquid capable of enacting with the pigment-based ink and capable of breaking the pigment dispersibility are employed in combination in order to solve the characteristic problems of a pigment-based ink and at the same time in order to utilize advantageous characteristics of the pigment-based ink. A recording process, in which a treatment liquid was applied so as to mix with a pigment-based ink on a printing medium in the liquid state either after or substantially at the same time of the application of the pigment-based ink onto the surface of the printing medium, was carried out as a part of the study. The quality of the resultant images was not necessarily satisfactory and in some cases the quality was rather found inferior to that of images formed using solely a pigment-based ink. Specifically, in the case where a pigment-based ink containing a pigment dispersed in an aqueous medium by a polymer dispersant was employed as a pigment-based ink in combination with a treatment liquid capable of reacting with the pigment-based ink, OD decrease attributed to the low area factor of the obtained ink dots was sometimes observed. The reason for occurrence of such a phenomenon is not clear, however, that is probably because the agglomeration of the pigment of the ink on the printing medium is significantly promoted by the treatment liquid. The area factor is, therefore, increased by increasing the shot-in ink quantity of a pigment-based ink to heighten OD, but in this case, fixing property is occasionally observed to be deteriorated.
A phenomenon 102 so called xe2x80x9cexudationxe2x80x9d or xe2x80x9chazexe2x80x9d as shown in FIG. 1 was sometimes observed in the dots (see 101 of FIG. 1) of a printing medium obtained by combining a pigment-based ink containing a self-dispersing pigment as a pigment-based ink with a treatment liquid to react with the pigment-based ink. FIGS. 2A, 2, and 2C illustrate the assumed mechanism of occurrence of this phenomenon.
When a pigment-based ink Ip containing a self-dispersing pigment but not containing a polymer dispersant is applied to a printing medium P (especially plain paper or the like) (see FIG. 2A) and then a treatment liquid S is applied thereon, generation of a reaction product starts (see FIG. 2B). As the reaction is proceeding, radial xe2x80x9cexudationxe2x80x9d occurs from approximately circular dots of the reaction product as illustrated in FIG. 2C and xe2x80x9chazexe2x80x9d surrounds the circumference of the dots. Such xe2x80x9cexudationxe2x80x9d or xe2x80x9chazexe2x80x9d is regarded as having the appearance of the phenomenon known as feathering, so that it causes deterioration of the printing quality.
The above-described xe2x80x9cexudationxe2x80x9d or xe2x80x9chazexe2x80x9d is assumed to be due to the following phenomenon. A dispersant-free pigment-based ink has a relatively high reaction rate in reacting with the treatment liquid, so that the dispersion break down of the dispersed pigment occurs at once to produce clusters of the reaction product, and at the same time a finely granular reaction product is also produced. The granular reaction product then flows out with penetration of the treatment liquid in a printing medium, so that the above-described xe2x80x9cexudationxe2x80x9d is supposedly caused.
As described above, in the case where a pigment based ink and a treatment liquid were simply combined with each other, phenomena occurred that the inventors did not expect and high quality ink-jet recording images were difficult to obtain. The inventors recognized it was necessary to develop an innovative technique in order to meet the objectives of overcoming the disadvantages of a pigment-based ink while effectively utilizing the advantages of the pigment-based ink by employing an ink-jet recording technique using a treatment liquid for the pigment-based ink.
The present invention is achieved in consideration of the foregoing newly obtained technical knowledge and findings and the objectives of the present invention are to provide an ink-jet printing apparatus and a printing method to obtain high quality printing employing the ink-jet recording technique using a treatment liquid for a pigment-based ink.
The ink-jet printing apparatus according to one embodiment of the present invention to meet the foregoing objectives applies an ink containing a pigment dispersed in an aqueous medium to a printing medium and then applies a treatment liquid to react with the ink, wherein the ink contains a first pigment of a self-dispersing pigment having at least one anionic group bonded directly or through another atomic group to the surface of the first pigment or a self-dispersing pigment having at least one cationic group bonded directly or through another atomic group to the surface of the first pigment, a second pigment capable of being dispersed in an aqueous medium by a polymer dispersant, and at least one dispersant selected from a polymer dispersant having the same polarity as that of the group bonded to the surface of the first pigment and a nonionic polymer dispersant; and the ink-jet printing apparatus comprises application means for applying the ink and the treatment liquid separately to a printing medium to mix the ink and the treatment liquid with each other in a liquid state on the printing medium.
The ink-jet printing apparatus according to another embodiment of the present invention applies an ink containing a pigment dispersed in an aqueous medium to a printing medium and then applies a treatment liquid to react with the ink, wherein the ink contains a first pigment of a self-dispersing pigment having at least one anionic group bonded directly or through another atomic group to the surface of the first pigment or a self-dispersing pigment having at least one cationic group bonded directly or through another atomic group to the surface of the first pigment, a second pigment capable of being dispersed in an aqueous medium by a polymer dispersant, and at least one dispersant selected from a polymer dispersant having the same polarity as that of the group bonded to the surface of the first pigment and a nonionic polymer dispersant; and the ink-jet printing apparatus comprises application means for applying the ink and the treatment liquid separately to a printing medium to mix the ink and the treatment liquid with each other in a liquid state on the printing medium followed by applying the ink to the ink and the treatment liquid mixed on the printing medium to further mix therewith on the printing medium in a liquid state.
The ink-jet printing apparatus according to still another embodiment of the present invention carries out printing by ejecting an ink containing a pigment dispersed in an aqueous medium to a printing medium and then ejecting a treatment liquid to react with the ink by using an ink-ejecting portion for ejecting the ink and a treatment-liquid-ejecting portion for ejecting the treatment liquid, wherein the ink employed therefor contains a first pigment of a self-dispersing pigment having at least one anionic group bonded directly or through another atomic group to the surface of the first pigment or a self-dispersing pigment having at least one cationic group bonded directly or through another atomic group to the surface of the first pigment, a second pigment capable of being dispersed in an aqueous medium by a polymer dispersant, and at least one dispersant selected from a polymer dispersant having the same polarity as that of the group bonded to the surface of the first pigment and a nonionic polymer dispersant; and the ink-jet printing apparatus comprises at least one pigment-based-ink-ejecting portion for ejecting the ink in the ink-ejecting portion, arrangement means for arranging the pigment-based-ink-ejecting portion and the treatment-liquid-ejecting portion in a prescribed relative position and control means for relatively moving the respective ejecting portions to the printing medium, and for having the respective ejecting portions eject the ink and the treatment liquid respectively so as to mix the ink and the treatment liquid on the printing medium.
The ink-jet printing process according to an embodiment of the present invention to meet the foregoing objectives includes a process of recording images on a printing medium which comprises a first step of applying an ink on a printing medium by employing an ink-jet recording process and a second step of applying a treatment liquid capable of reacting with the ink, wherein the ink contains a first pigment and a second pigment dispersed in an aqueous medium, the first pigment being a self-dispersing pigment having at least one anionic group bonded directly or through another atomic group to the surface of the first pigment or a self-dispersing pigment having at least one cationic group bonded directly or through another atomic group to the surface of the first pigment and the second pigment being capable of being dispersed in an aqueous medium by a polymer dispersant; the ink further contains at least one dispersant selected from a polymer dispersant having the same polarity as that of the group bonded to the surface of the first pigment and a nonionic polymer dispersant; and the second step is conducted subsequently to the first step or substantially simultaneously with the first step so as to bring the ink and the treatment liquid into contact with each other in a liquid state on the printing medium.
By the invention according to the respective embodiments as described, high quality images with extremely high OD and high edge sharpness can be obtained and various desirable properties such as abrasion resistance and fixing properties can be improved. The reason for such effects by subsequent or substantially simultaneous application of the treatment liquid with respect to application of the ink containing the first pigment and the second pigment is not clear. The following facts have, however, been observed by the inventors from various experiments relating to the present invention. That is, when an ink containing the first pigment and the second pigment is applied to a printing medium, dots of the ink with certain surface areas are formed on the surface of the printing medium P as illustrated in FIG. 3A. The size (the diameter: d1) of the ink dots is larger than the size (the diameter: d2) of dots of a conventional pigment-based ink (an ink containing a pigment dispersed by a polymer dispersant or an ink containing a self-dispersing pigment) as illustrated in FIG. 3B (d1 greater than d2). The reason why such a phenomenon is observed is not clear but it is assumed to be due to the following mechanism. That is, the second pigment on which the polymer dispersant has been adsorbed and the first pigment are electrically repulsed from each other in an ink so that the agglomeration force of the pigments in the ink gets weak as compared at least with that of an ink containing only a polymer-dispersant-dispersed pigment. In the case where such an ink is printed on a paper surface, the coloring materials in the ink hardly penetrate the paper in the thickness direction since the polymer dispersant has been adsorbed on the second pigment. On the other hand, in the horizontal (transverse) direction of the paper surface, the pigment is strongly agglomerated in the case of an ink containing a second pigment and a polymer dispersant since polymer molecules are rapidly entangled with one another with penetration of the solvent of the ink in the paper and decrease in water by evaporation or since cross-linking of the polymer with the pigment is caused. On the contrary, entangling and cross-linking of the polymer are prevented or suppressed owing to coexistence of a first pigment in the case of the ink of the embodiments so that the strong intermolecular force of pigments in the ink is moderated by the repulsion of the first pigment and the polymer dispersant. Thus, the ink is easily diffused in the transverse direction of the paper surface, and though the diffusion is moderated, the diffusion is believed not to be in disorder owing to the effect of the agglomeration force of the pigments.
When a treatment liquid S is applied to the ink dots evenly and widely diffused on the surface of a printing medium (see FIG. 2B and FIG. 2C), a reaction is caused in the interface of the ink and the treatment liquid. As described above, owing to the wide diffusion of the ink dots, reaction points with the treatment liquid are numerous as compare with those in the case of a conventional ink, and moreover, owing to the enlargement of the ink dots, the thickness (t1) of the ink dots is small as compared with the thickness (t2) of conventional ink dots on the surface of the printing medium so that the reaction with the treatment liquid is supposed to be completed within a short time. Consequently, it can be presumed that fixing time shortening, fixing property improvement as well as edge sharpness improvement of the ink dots are achieved in the embodiments of the present invention. According to the foregoing mechanism, it can clearly be understood that the effects of the embodiments are characteristically derived from the system where an ink is applied to a printing medium before or practically simultaneously with application of a treatment liquid.
For the embodiments of the present invention, in the case where a treatment liquid having excellent penetration properties to a printing medium is used, the fixing property and the edge sharpness of the ink dots are further improved. That is probably because a solvent including water becomes more penetrative and penetrates the printing medium owing to the penetrating force of the treatment liquid while the ink and the treatment liquid are reacting with each other. Generally, in the case where a coloring material penetrates a printing medium, optical density is usually decreased; nevertheless, in the case where an ink is applied prior to the application of the treatment liquid just as in the embodiments, pigment rarely penetrates the printing medium to such a degree that a decrease in OD is caused. A coloring material is rather made likely to remain in the surface and its periphery of a printing medium by the reaction with the treatment liquid, and as a result, OD is increased to a greater extent than that in the case where no treatment liquid is used.
Further, for the embodiments, it is preferable to use a treating liquid of which the components are optimized according to the type and the ratio of a first pigment and a second pigment in the ink to further heighten the image quality. That is, a self-dispersing pigment probably has a mine-like shape having a large number of whisker-like polar groups (anionic groups) as in the circumference of the pigment core as illustrated as a model in FIG. 4A. On the other hand, a polymer compound, for example, polyallylamine (PAA) having a large number of cationic groups in one molecule is generally illustrated as FIG. 4B. In the case where such compounds are mixed with a self-dispersing pigment, as illustrated in FIG. 5, a PAA polymer is entwined around the circumference of the self-dispersing pigment. The cationic group of PAA, however, cannot geometrically be bonded to every anionic group, and as a result, the reaction product of the self-dispersing pigment and PAA probably is in a cationic state as a whole. Thus, particles of a substance obtained by mutual reaction of pigment particles with a small diameter and PAA probably have weak intermolecular force and are liable to electrically repulse one another and are difficult to agglomerate to form a large complex. Consequently, it can be assumed that the fine particles are the cause of haze-like bleeding in the circumferences of dots. On the contrary, regarding a pigment dispersed by a polymer dispersant, the polymer dispersant itself has a large number of anionic groups or cationic groups. Accordingly, even when a compound having one cationic group or one anionic group in one molecule is included in the treatment liquid, the dispersibility of the polymer dispersant is not completely broken. Hence, by employing a treatment liquid containing a cationic polymer compound such as PAA and a cationic low molecular weight compound such as benzalkonium chloride in a prescribed ratio for an ink containing a first pigment having an anionic group bonded to the surface and a pigment dispersed by an anionic polymer dispersant, the dispersibility of the respective pigments in the ink is reliably broken on a printing medium so that production of unreacted cationic groups, which are a cause of haze, can be suppressed as much as possible. As a result, images with extremely high quality, high OD, no haze, and excellent fixing properties, can be formed on a printing medium within a short fixing time.
The ink-jet printing apparatus according to another embodiment of the present invention applies an ink containing coloring materials to a printing medium and then applies a treatment liquid for making the coloring materials of the ink insoluble, wherein the ink contains a first ink containing either a self-dispersing pigment having at least one anionic group bonded directly or through another atomic group to the surface of the first pigment or a self-dispersing pigment having at least one cationic group bonded directly or through another atomic group to the surface of the first pigment, a second ink containing a pigment capable of being dispersed in an aqueous medium by a polymer dispersant and at least one dispersant selected from a polymer dispersant having the same polarity as that of the group bonded to the surface of the first pigment and a nonionic polymer dispersant; and the ink-jet printing apparatus comprises application means for applying the first ink, the second ink, and the treatment liquid separately to a printing medium to mix the first ink, the second ink, and the treatment liquid in a liquid state on the printing medium.
The ink-jet printing apparatus according to another embodiment of the present invention carries out printing, using an ink-ejecting portion for ejecting an ink containing coloring materials to a printing medium and a treatment-liquid-ejecting portion for ejecting a treatment liquid for making the coloring materials of the ink ejected by the ejecting portion insoluble, by ejecting the ink to a printing medium and then ejecting the treatment liquid, wherein the ink-ejecting portion comprises a first ink-ejecting portion for ejecting a first ink containing either a self-dispersing pigment having at least one anionic group bonded directly or through another atomic group to the surface of the first pigment or a self-dispersing pigment having at least one cationic group bonded directly or through another atomic group to the surface of the first pigment and a second ink-ejecting portion for ejecting a second ink containing a pigment capable of being dispersed in an aqueous medium by a polymer dispersant and at least one dispersant selected from a polymer dispersant having the same polarity as that of the group bonded to the surface of the first pigment and a nonionic polymer dispersant; and the ink-jet printing apparatus comprises arrangement means for arranging the first ink-ejecting portion, the second ink-ejecting portion, and the treatment-liquid-ejecting portion in a prescribed relative position and control means for relatively moving the respective ejecting portions to the printing medium, and for having the respective ejecting portions eject the first ink, the second ink, and the treatment liquid respectively so as to mix the first ink, the second ink, and the treatment liquid on the printing medium.
The ink-jet printing process according to a further embodiment of the invention includes a process of recording images on a printing medium, the process comprising the steps of:
(i) applying an ink on a printing medium by employing an ink-jet recording process; and
(ii) applying a treatment liquid capable of reacting with the ink, wherein
the ink contains a first pigment and a second pigment dispersed in an aqueous medium, the first pigment being selected from a self-dispersing pigment having at least one anionic group bonded directly or through another atomic group to the surface of the first pigment and a self-dispersing pigment having at least one cationic group bonded directly or through another atomic group to the surface of the first pigment and the second pigment being capable of being dispersed in an aqueous medium by a polymer dispersant,
the ink further containing at least one dispersant selected from a polymer dispersant having the same polarity as that of the group bonded to the surface of the first pigment and a nonionic polymer dispersant, and wherein the step (ii) is conducted subsequently to the step (i) or substantially simultaneously with the step (i) so as to bring the ink and the treatment liquid into contact with each other in a liquid state on the printing medium.
The ink-jet printing process according to the first embodiment of the present invention comprises the steps of:
respectively applying a first ink, a second ink, and a treatment liquid reactable with at least one of the first and the second ink to the surface of a printing medium in such a manner that the respective first ink, second ink and treatment liquid are brought into contact with each other on the surface of a printing medium in a liquid state, wherein the first ink contains either a self-dispersing pigment having at least one anionic group bonded directly or through another atomic group to the surface of the first pigment or a self-dispersing pigment having at least one cationic group bonded directly or through another atomic group to the surface of the first pigment; the second ink contains a pigment capable of being dispersed in an aqueous medium by a polymer dispersant and at least one dispersant selected from a polymer dispersant having the same polarity as that of the group bonded to the surface of the self-dispersing pigment and a nonionic polymer dispersant; the treatment liquid contains a compound having the opposite polarity to that of the group bonded to the surface of the self-dispersing pigment; and the treatment liquid is applied after application of at least one of the first ink and the second ink to a printing medium.